1. Field of the Invention
This invention relates to a retention-type electrophotographic recording apparatus producing a number of copies of the original from an electrostatic latent image, which is formed once on a photoreceptor, by repetition of the development of the latent image with a toner and transfer of the toner image to a transfer paper.
2. Description of the Prior Art
As an electrophotographic method for the production of two or more copies from an electrostatic latent image which is formed once on a photoreceptor, a roller-transfer method for conducting transfer of the toner image to a transfer paper using rollers and an ionic beam modulation method using a screen photoreceptor have been proposed.
According to the roller-transfer method, the transfer paper is pressed against the toner image on the photoreceptor by a conductive gum roller or a roller having an dielectric layer on the surface thereof, to which a high pressure is applied. The roller mainly serves to prevent electrical destruction and/or contamination of the latent image on the photoreceptor.
On the other hand, according to the ionic beam-modulation method, an ionic beam is modulated based on a first latent image on a screen photoreceptor having a slit to form a second latent image, which is then developed with a toner. The latent image is retained for successive development and transfer to obtain the necessary number of copies.
Both of the above-mentioned conventional transfer methods can provide a plurality of copies by the repeated use of an latent image formed once on a photoreceptor until the potential of the latent image decays to a certain level.
However, in the roller transfer method, the adhesion of toner to the surface of the roller occurs resulting in non-uniformity of solid areas and/or the formation of white spots in solid areas. The roller must uniformly press the transfer paper to the photoreceptor and is sometimes directly brought into contact with the photoreceptor causing damage to the photoreceptor. In order to eliminate damage to the photoreceptor, resulting in unacceptable and/or indistinct images, the apparatus required for the roller transfer method must have a complicated structure.
Similarly, the ionic beam modulation method requires not only a large scale-apparatus but also troublesome processes. The production of the apparatus required for the ionic beam modulation method is quite difficult and is expensive.
To eliminate the above-mentioned drawbacks, a corona transfer method using a DC corona charger can be employed, wherein the back face of the transfer paper which has been in contact with a toner image on a photoreceptor is corona-charged with charges having a different polarity from the charging polarity of the toner and, due to the resulting electric field therebetween, the toner image is transferred to the transfer paper. The electric charges injected into the transfer paper in the transfer step of the toner image to the transfer paper create a potential in the region of the latent image corresponding to the background in the original, causing the increase in a fog density from the second copy. The electric charges also bring about the adhesion of the transfer paper to the photoreceptor to cause a decrease in capacitance of the transfer paper against the ground and the steep rise of the transfer paper potential in the succeeding separation step of the transfer paper, resulting in an electric discharge therebetween so that the latent image on the photoreceptor is electrically contaminated and provides unacceptable copies. Although the rise of the potential in the region of the latent image corresponding to the background in the original can be avoided by the transfer of electric charges to the photoreceptor in the transfer step at a decreased transfer output power, the electrical contamination of the latent image due to electrical discharge is not eliminated as the transfer paper is insufficiently adhered to the photoreceptor to result in an insufficient transfer of the toner image thereto.
To improve such an insufficient transfer of the toner image to the transfer paper, the back face of the transfer paper can be subjected to a discharge treatment just after the transfer of the toner image in order to decrease the transfer paper potential. However, the transfer region must be close to the discharge region and the DC output in the transfer region must be reduced, so that a stable transfer of the toner image to the transfer paper cannot be carried out.